Prepare the Meat: Process the beef, garlic, and ginger into a bowl. Season with soy sauce, sugar, sesame oil, and black pepper. Mix well.
Cook Noodles: Boil the dangmyeon noodles for 7-8 minutes, then rinse in cold water and chop into small pieces.
Prepare Vegetables: Mince the onion and buchu. Chop the fermented kimchi into small pieces.
Prepare Tofu: Squeeze out the tofu slightly to remove some liquid but keep it juicy.
Mix Filling: Combine the noodles, onion, buchu, kimchi, and tofu in a bowl. Season with salt, sesame oil, and ground black pepper. Mix well using hands.
Assemble Dumplings: Place the filling on each mandu wrapper, fold and seal to form the dumplings.
Cooking: Cook the dumplings by your preferred method (steaming, boiling, or pan-frying). Cooking times may vary based on the method.